This invention relates to a coaxial cable having excellent transmission characteristics for a high-frequency signal and high dielectric strength, and also relates to a transmission transformer using this coaxial cable.
A transmission transformer, used in the transmission of signals between medical equipments, is required to have high dielectric strength, and also is required to have enhanced transmission characteristics for high-frequency signals so as to meet the high-speed processing of signals by recent computers. These demands are not limited to medical equipments, but there are similar demands for other equipments. And besides, there is a demand for a compact and lightweight design of such a transformer.
As shown in FIG. 7, Japanese Utility Model Publication No. 63-140609U discloses a transmission transformer 101 for transmitting a signal, in which two conductive wires 104 and 105 are wound on a toroidal core 102. Each of the conductive wires 104 and 105, wound on the toroidal core 102 is a polyurethane-coated copper wire. Each of the conductive wires wound on the toroidal core of the transformer comprises a conductive single wire covered with an insulating layer.
On the other hand, as shown in FIG. 8, Japanese Patent Publication No. 6-203664A shows a coaxial cable for transmitting a high-frequency signal. In the coaxial cable, an insulative resin (polytetrafluoroethylene) 112 is coated on a center conductor 111 (composed of a silver-plated copper-sheathed steel wire), and a copper-deposited aluminum polyester tap 113 is wound on this resin coating layer 112. A plurality of tinned soft copper wires 114 are spirally side-wound on an outer periphery of the tape layer 113. The tinned soft copper wires are immersed in molten tin to form a tin coating 115. A protective layer 116 is formed so as to cover the tinned soft copper wires 114 coated with the tin coating 115.
International Standard for the Safety of Medical Electrical Equipment (IEC 60601-1) require that the insulation between a primary winding and a secondary winding of a transformer, used in a medical electrical equipment and required to have a reinforced insulation or a double insulation, should have the following construction so as to secure a required withstand voltage (dielectric strength).    (1) When one insulating layer is provided, its thickness must be not smaller than 1 mm.    (2) When two insulating layers are provided, the sum of their thicknesses must be not smaller than 0.3 mm.    (3) When three insulating layers are provided, a combination of any two of the three layers must pass a withstand voltage test for a reinforced insulation.
In the case of the transmission transformer disclosed in Japanese Utility Model Publication No. 63-140609U, each of the two conductive wires, wound on the toroidal core, comprises a single wire coated with a single insulating layer, and these conductive wires do not exhibit sufficient dielectric strength and high-frequency characteristics.
In the case of the coaxial cable disclosed in the Japanese Patent Publication No. 6-203664A, the center conductor is coated with a single insulative layer having a thickness which does not meet the requirements of the IEC 60601-1. Further, since the step of coating the molten tin is necessary, the manufacturing cost accordingly increases.